The object of the present invention is a pulp drying line and a method for drying pulp, as defined in the preambles of the independent claims presented below.
It is previously known to dry chemical pulp by means of drying cylinders, in which case a conventional web with a dry matter content of 45-50% is formed from the pulp coming from the headbox on the wire and press section, the said web being finally dried to the desired dryness on the cylinder dryer section. It is, however, difficult to break up this pulp again in connection with paper manufacture.
To remedy the problem, it is proposed in Finnish patent number 58020 that the pulp web coming from the wire and press section be dried to a dry matter content of only approximately 60-70% in the conventional manner with cylinders, after which the actual final drying is proposed to be carried out by means of hot gas blown through the pulp. This means that the conventional pre-dried pulp web coming from the cylinders will have to be shredded and defibrated to form a loose pulp which allows through-blowing. Of the loose pulp thus obtained a porous air-permeable pulp web is formed between two horizontal wires, and the said web is finally dried. Shredding and defibration of the viscous pre-dried pulp web into a fine loose pulp is not, however, always easy.
In the case of FI 58020, final drying takes place gradually, so that in the first part of the dryer section hot gas is blown through the pulp web from the blow box above the web to the suction box below the web, from where the exhaust air is conducted, via a heater, to the blow box adjacent to the suction box below the web and second in order, from which air is blown further up through the web to the suction box second in order above the web. From the second suction box the air is blown further in a corresponding manner through a third pair of blow/suction boxes, etc. At the end of the dryer section, the exhaust air is taken from the last suction box back to the beginning of the dryer section through an air duct having a length corresponding to the entire dryer section. In this case, therefore, the air circulation is very long, since the entire drying air flow passes first in a wave-like fashion through the long dryer section and then finally back to the beginning of the dryer section. Any changes in the air flow at the beginning of the air circulation will affect air flow throughout the entire dryer section. The system does not allow for local adjustment of air blowing.
The pulp drying solution described above, in which pre-drying is carried out by means of drying cylinders and final drying in a horizontal dryer section, requires a conventional web to be formed of the pulp at two separate timesxe2x80x94first, before cylinder drying, and second, before fan drying. The drying line with its cylinders and horizontal final dryer section is long and thus takes up a lot of space.
The aim of the present invention is to achieve an improved drying line and method for drying pulp.
A particular aim in this case is to achieve a drying line construction which takes up less space and is more economical as regards its construction costs.
A further aim is to achieve a method by means of which still relatively moist pulp coming from the pulp press can be dried efficiently without conventional pre-drying by means of drying cylinders.
To achieve the above aims, the method, apparatus and chemical pulp drying line relating to the invention are characterised by what is defined in the characterising part of the independent claims presented below.
A typical pulp drying line relating to the invention for drying chemical pulp with a dry matter content of less than 3% comprises a pulp press, a defibrator, a web forming section and the actual dryer section. The pulp press used may, for example, be a roll press in which the pulp is pressed in a nip between two rolls to a dry matter content of about 30-50%, typically 40-50%. From the pulp press the pulp is taken, for example, by means of a screw conveyor, to the defibrator. According to a preferred solution of the invention, the pulp is taken directly from the press to the defibrator without being formed into a web in the meantime, which thus saves the cost of web forming at this stage. The pulp can be conveyed by means of the screw conveyor, which means that the pulp can also be dewatered at this stage.
The pulp pre-dried in the defibrator is defibrated to form a loose pulp suitable for web formation. The loose pulp is taken to the web forming section, where the pulp is formed into an air-permeable pulp web on a forward-running air-permeable forming base, typically a wire. The loose pulp flakes and/or fibres are spread by spreading means on the forming base, to form an air-permeable layer.
In the actual dryer section, which comprises a forward-running air-permeable support fabric, such as a wire, which supports the pulp web as it passes through the dryer section, and blowing means or the like, by means of which drying air or gas is blown through the pulp web in order to dry it.
A typical chemical pulp drying line relating to the invention comprises, in the actual dryer section,
blow boxes, through-blowing cylinders such as drying drums, or other similar means for blowing drying air through the pulp web to be dried, and
suction boxes, suction cylinders or air-removal means for removing the air that has passed through the pulp web and absorbed moisture into it from the vicinity of the pulp web.
In a first solution relating to the invention, blow boxes and suction boxes are fitted in the dryer section to form narrow vertical drying gaps extending through the dryer section in cross-web direction. The gaps are only about 30-100 mm, preferably 50-70 mm, wide in the machine direction. The drying gaps are delimited on one side by blow boxes fitted across the machine, and on the other by one or more suction boxes extending across the machine.
The dryer section preferably incorporates several drying gaps in succession, in which case the pulp web to be dried is arranged to pass through the successive drying gaps alternately upwards and downwards. At the same time, hot drying air is blown from a blow box on the first side of the web to the opposite suction box on its other side. The drying air is supplied to the blow boxes from fan towers adjacent to the actual dryer section. A pressure difference of typically about 200-800 Pa is arranged across the pulp web, due to which the drying air flows through the web.
While passing through the web, the drying air cools down and becomes wet. From the suction box the cooled air containing moisture is taken to the heater in the fan tower, from where the air, once heated, is taken back to the blow box on the first side of the web. Some of the wet cooled air coming from the suction box is replaced by fresh dry air.
In this first solution relating to the invention, the drying air circulation has been arranged so as to be local, and is thus very short. Drying air circulation takes place by means of fan towers integrated into the dryer section. The fan towers supply drying air to the blow boxes from single vertical compressed air chambers connected to the ends of the blow boxes and situated adjacent to the machine. A compressed air chamber is preferably arranged to supply drying air to two separate blow box units fitted in succession in the machine direction and comprised of blow boxes placed on top of one another.
The fan towers collect the return air from the suction boxes into a return air chamber connected to the ends of the suction boxes and fitted adjacent to the dryer section, thus maintaining underpressure in the suction boxes. One return air chamber is preferably arranged to collect return air from two separate suction boxes fitted in succession in the machine direction.
In this first dryer section relating to the invention, several pressure chambers and vacuum return air chambers are typically arranged alternately and in succession on both sides of the machine. The pressure chambers and return air chambers on the different sides of the machine are fitted opposite each other in the dryer section, so that drying air is supplied to the drying gap from one side of the machine, while removing return air from the other side of the machine. At least some of the return air supplied to the return air chamber is recirculated via the heater back to the pressure chamber. From one fan tower, drying air is typically supplied to 3-5 drying gaps. In the solution relating to the invention separate long air ducts or air systems outside the dryer are not required, as the drying air circulation has been arranged so as to be local.
The travel of the pulp web is supported in this first solution relating to the invention usually by means of at least one wire as it passes through the vertical drying gaps between the blow and suction boxes. However, through the first drying gap or gaps the pulp web is preferably conveyed between two wires. If necessary, the pulp web can be conveyed through the entire dryer section supported by two wires. On the other hand, in some cases the pulp web may reach the necessary strength towards the end of the dryer section to be able to run without the support of the wire. Turning rolls or the like, by means of which the travel of the pulp web and the wire can be turned from one drying gap to the next, are arranged between the drying gaps, in the upper and lower parts of the dryer section. The turning rolls can also be utilised for the purpose of drying by providing them with suitable suction or blowing. Vacuum rolls at the same time ensure the travel of the pulp web.
In the drying line relating to the invention, in front of or at the start of its actual dryer section, a horizontal web forming section is typically arranged, in which web forming section a pulp web is formed of the loose pulp by spreading the pulp on a horizontal air-permeable forming base, such as a wire, or by feeding pulp into the gap between two wires. In the horizontal section, blow boxes blowing drying air may be fitted above or below the pulp web supported by two wires, and on the other side of the web, opposite the blow boxes, may be fitted a suction box or boxes for removing the air blown through the web as exhaust air from the web. In the horizontal section, the pulp web is pre-dried to a dry matter content of, for example, about 55-65%.
After the horizontal run, the web is guided preferably first downwards between two wires, to the lower part of the dryer section, and from there on, turned by the turning roll, to the lower end of the first long drying gap, that is, a drying gap of full height. From there the pulp web can be conveyed upwards, supported by one or two wires, in the drying gap delimited by a blow box or boxes and a suction box or boxes. If only one conveying or support wire is used, this is fitted to run between the suction box and the pulp web, which means that it will prevent the web from being sucked into contact with the suction box. In the upper part of the drying gap, the wire conveys the pulp web over the second turning roll to the next drying gap in the direction of the web, in which gap the pulp web travels downwards.
In this first drying line relating to the invention, the pulp web is typically made to pass through  greater than 10, e.g. about 20-30, vertical drying gaps, supported first by two wires and later by only one wire.
In an alternative second drying solution relating to the invention the air-permeable pulp web may be dried by means of drying drums in which are fitted means for blowing or sucking drying air through the pulp web passing over the drum shell. The drum is preferably of light-weight construction. Its outermost shell may be made, for example, of strong netting or perforated plate, which allows air to be blown or sucked through the shell. That part of the drum which is not covered by the pulp web may be sealed either from the inside or the outside of the drum, if desired. In some cases the drum shell may even be left unsealed. The drying drums may be drawn by the wire, in which case they will not require driving units for to effect traction.
The drying drums may be fitted on top of one another in vertical xe2x80x9cstacksxe2x80x9d, as has been done with cylinder dryers. On the other hand, drying drums may be fitted in succession, in the same way as drying cylinders in a conventional paper machine dryer section.
All drying drums may be through-blowing drums, that is, provided with means which blow drying air through the pulp web passing over the drum shell. In this case the pulp web, which typically passes between two wires, is arranged to run alternately with the first and second side towards the drying drum, which means that the direction of flow of the drying air through the pulp web changes each time when changing from one roll to the next. If so desired, at least some of the drying drums may be equipped with means for effecting suction, which means that air is sucked through the pulp web towards the drum.
When the drying drums act as blowing drums, the drying air that has flowed through the pulp web may be released freely into the area surrounding the drying drums. The drying drums are preferably surrounded by a hood, so that the used drying air can be recovered, heated and re-used for drying. Air is supplied to the drying drums preferably on the driving side, at the ends of the drums. Some air is removed from the hood as exhaust air and replaced by the required amount of replacement air.
If necessary, a pulp web pre-dried in the conventional manner, by pressing in web form, can be dried further according to the methods described above, provided that the permeability of the pulp web is increased before drying by forming apertures in the web to create pores. By means of the perforation it is ensured that the heat transfer area between the drying air and the pulp is sufficiently large. The heat transfer surface should preferably cool the temperature of the drying air close to the temperature of the pulp web to be dried during the through-flow. The perforation makes it possible to dry a pulp web with a higher than usual grammage. The grammage of a chemical pulp web may, for example, be 600-4000 g/m2, most preferably 1000-3000 g/m2.
The perforation percentage, that is, the open surface area, may be 2-20%, most preferably 5-15%. The diameter of the holes is typically 0.3-8 mm, most preferably 0.5-4 mm, preferably 1-3 mm. The distance between holes is 1-10 mm, most preferably 2-5 mm. Perforation is preferably carried out in a roll nip, the said nip being formed between two rolls, one of which is provided with a perforation surface pattern, and the other is a soft roll. The actual perforation may take place by blowing compressed air jets through the web while the web is supported against the wire or roll.
The perforation described above can also be utilised also to increase the efficiency of conventional pulp web air drying in cases where effective contact is required between the drying air and the pulp, for example, in conventional horizontal or vertical drying of chemical pulp.